The present invention relates to a method and apparatus for electrically discharging conductive workpieces which pass through electrostatic lacquering or coating installations and are lacquered or coated by spray guns or the like.
The invention will be described with reference to spray coating since this constitutes the main field of application for the present invention; the invention can, however, also be used for electrostatic lacquering systems.
During electrostatic coating, forces created in the electrostatic high voltage field are utilized for transporting the powder to be applied to the workpiece. Between the electrically negative spray gun and the grounded workpiece there exists a voltage of about 100 to 150 kV and thus a field of high electrical intensity. The powder particles disposed within the electrical field are negatively charged and are thus moved by the electrical field forces along the lines of force of the field toward the workpiece. In order for the driving potential difference between the electrostatically charged powder particles and the workpieces to remain effective, it is important for the workpieces to be well grounded. Furthermore, grounding of the workpiece prevents it from being electrically charged by the coating process and thus endangering the safety of the operating personnel.
If high electrical charges exist on the workpiece, spark discharges will occur when the workpiece comes close to grounded objects, possibly resulting in explosive combustion of the mixture of powder and air in the spray chambers. It is therefore necessary to assure sufficient grounding of the workpieces under all circumstances.
It is known to monitor the ground connection which has been provided by effecting a direct measurement of the resistance between the workpiece and ground. This requires contact with the workpiece, however, in order to form an ohmic measuring circuit. In many cases this cannot be achieved in practice due to the shape of the workpiece involved. Furthermore, substantial measuring errors result if the point of contact -- be it at the workpiece or at the measuring sensor -- is soiled.
Test have been performed in which a workpiece was conducted through a high voltage field before entering the coating system, was there charged electrically and then, after a certain period of time, a measurement was made to determine how much charge was retained by the workpiece.
Thus in the known processes the grounding resistance is measured directly. If this resistance is still too high it is necessary to remove the workpieces involved or to interrupt the process until a satisfactory ground connection has been produced.